For an uplink channel of LTE-A (LTE-Advanced), which is an evolved version of 3rd generation partnership project long-term evolution (3GPP LTE), using “non-contiguous frequency transmission” in addition to contiguous frequency transmission is under consideration to improve sector throughput performance (see Non-Patent Literature 1).
Non-contiguous frequency transmission is a method of transmitting a data signal and a reference signal by assigning such signals to non-contiguous frequency bands, which are dispersed in a wide range of band. As shown in FIG. 1, in non-contiguous frequency transmission, it is possible to assign a data signal and a reference signal to discrete frequency bands. Therefore, in non-contiguous frequency transmission, compared to contiguous frequency transmission, the flexibility in assigning a data signal and a reference signal to frequency bands in each terminal increases. By this means, it is possible to gain greater frequency scheduling effects.
As a method of reporting frequency resource assignment information for non-contiguous frequency transmission from a base station to a terminal, there is a method of reporting whether or not to perform assignment for each resource block group (RBG) in the system band, using a bitmap (see Non-Patent Literature 2). As shown in FIG. 2, a base station reports to a terminal subject to frequency assignment using one bit whether or not to assign frequency resources per predetermined RBG (per four [RBs] in FIG. 2). That is, in a plurality of RBGs formed by dividing the system band per predetermined RB, including an RBG that is assigned to a terminal subject to frequency assignment (hereinafter referred to as “assigned RBG”) and an RBG that is not subject to assignment (hereinafter referred to as “RBG not assigned”), a base station reports to a terminal subject to frequency assignment, a frequency assignment bit sequence that is obtained by assigning the bit value of 1 to one of the above RBGs and assigning the bit value of 0 to the other. In FIG. 2, the RBG to which bit “1” is assigned is frequency area assigned to a terminal subject to assignment while the RBG to which bit “0” is assigned is frequency area that is not subject to assignment to the terminal subject to assignment. Therefore, the number of signaling bits required for frequency resource assignment information matches the number of RBGs in the system bandwidth.
In LTE, as shown in FIG. 3, the size of an RBG (=P) varies depending on the system bandwidth (see Non-Patent Literature 3). As shown in FIG. 3, a greater size of an RBG is used for the broader system bandwidth, reducing the number of signaling bits.
Further, in LTE, a sounding reference signal (SRS) of an uplink channel is used. Here, “sounding” means estimation of channel quality. An SRS is transmitted by time-multiplexing data on a specific symbol, mainly to perform estimation of the channel quality indicator (CQI) of an uplink channel data channel.
Further, among the methods of transmitting SRSs are a method of transmitting SRSs in the transmission bandwidth as broad as the system bandwidth (i.e. method of transmitting SRSs in a broad band), and a method of transmitting SRSs in which SRSs are transmitted in a narrow band at each transmission timing by changing transmission frequency bands in time sequence (that is, by performing frequency hopping) (i.e. method of transmitting SRSs in a narrow band). When the broad-band SRS transmission method is used, CQIs are estimated over a broad band at one time. Further, when the narrow-band SRS transmission method is used, CQIs are estimated over a broad band by using several SRSs transmitted in a narrow band.
Generally, path loss for a signal that is transmitted from a terminal near the cell border and is received by a base station, is significant. Further, because the maximum transmission power of a terminal is limited, in the case of the broad-band SRS transmission, reception power of a base station per unit frequency lowers and the reception SINR lowers. As a result of this, the accuracy of CQI estimation deteriorates. Therefore, for a terminal near the cell border, the narrow-band SRS transmission method for performing transmission so as to focus limited power on a predetermined frequency band, is employed. In contrast, path loss for a signal that is transmitted from a terminal near the cell center and is received by a base station, is small. For this reason, even when the broad-band SRS transmission method is employed, is possible to fully secure reception power of a base station per unit frequency. As a result of this, the broad-band SRS transmission method is employed for a terminal near the cell center.
Further, in LTE, the transmission bandwidth of the broad-band SRS transmission method is set N times (N is an integer) as broad as the transmission bandwidth of the narrow-band SRS transmission method, so as to use the same frequency band in which SRSs can be transmitted (i.e. sounding band, or frequency band with which CQI estimation can be performed), regardless of the broad-band SRS transmission method or the narrow-band SRS transmission method. That is, when the narrow-band SRS transmission method is employed, CQIs of the same frequency band as the frequency band in the broad-band SRS transmission method are estimated by applying frequency hopping N times. Specifically, in LTE, the minimum bandwidth for transmitting SRSs is four RBs, and all of the transmission bandwidths of SRSs are formed with RBs of multiples of four (see Non-Patent Literature 4).